One of the problematical points in a spread spectrum communication system using a wide band frequency region is that communication is made impossible or error rate is increased by high level narrow band interference. In order to solve this problem, a filter using surface acoustic wave (hereinbelow abbreviated to SAW) has been proposed.
For example, the inventor of this invention has proposed an SAW device indicated in FIGS. 7, 8 and 9 in U.S. Ser. No. 521,142 and U.S. Ser. No. 637,058 in order to intend to improve characteristics of a system adaptable for suppressing narrow band interference, which is useful for the spread spectrum communication.
In the figures, reference numeral 1 is a p' type Si monocrystal substrate; 2 is a p type Si epitaxial layer; 3 is a thermal oxide layer; 4 is a ZnO piezo-electric layer; 5, 6 and 7 are metal electrodes, which are an input transducer, an output transducer and a gate electrode, respectively; 8 is a p' type high impurity concentration diffusion region formed within the epitaxial layer 2 under the input transducer and the output transducer; 9 is an n' type impurity diffusion region formed within the epitaxial layer 2 under the gate electrode; 10 is an n' type impurity diffusion region formed within the epitaxial layer 2 outside of the input transducer 5; and 7' is a gate electrode formed on the piezo-electric layer, corresponding to the n' type impurity diffusion region. A first PN diode array is formed along the SAW propagation path by the n' type impurity diffusion region 9 stated above and a second PN diode array is formed along the SAW propagation path by the n' type impurity diffusion region 10. Several channels, each having the construction described above, are formed. 11 is a resistor connected with the PN diode array 9 or 10; 12 is a DC power supply; and 13 is a voltage signal monitor terminal, through which a voltage signal obtained by converting an input signal into SAW and detecting it by the second PN diode array, the intensity (electric power) of the input signal within the relevant channel (frequency region) being observed as variations in the voltage. Further 14 is a bias control terminal for the first PN diode array.
In the device disclosed in the older application U.S. Ser. No. 521,142 a function of monitoring the spectrum intensity of the input signal is added to a prior art element. In this construction described in the older application, it is necessary that the epitaxial layer is sufficiently thick for obtaining a great variable width of propagation loss of the SAW in the filter portion between the input and the output transducer. However, when the epitaxial layer is thick, it has a drawback that the propagation loss in the detection portion is great, which decreases the intensity of the detected signals. In order to solve this problem, the inventor of the present invention has proposed a construction, in which the epitaxial layer is thick in the filter portion and thin in the detecting portion, as indicated in FIG. 9. Owing to this construction, the detection sensitivity was remarkably improved without giving any influences on filter characteristics. However, since the propagation loss of the SAW due to electro-acoustic (AE) effects, which are interactions between carriers in Si and the potential of the SAW, is not removed perfectly and worsening in the detection sensitivity can not be neglected, there remains still a room for improvement.